1. Field of the Invention
This invention relates to a method for preparing supports for planographic printing plates, and, in particular, to the one comprising electrolytic surface graining of an aluminum plate utilizing an alternating charge (a.c.) current of specifically regulated wave form.
2. Description of the Prior Art
A typical support for planographic printing plates is aluminum, the surface of which is customarily grained for various reasons, including the improvement of adhesion to the photosensitive coating to be provided thereon, the provision of a high level of water-retention, etc. Such surface graining can be formed by a variety of techniques, including so-called electrochemcial graining (i.e., electrolytic etching) of aluminum plate surfaces. Electrolytic etching can be carried out in an electrolytic cell filled with an acidic electrolyzing solution by passing an alternating current between the aluminum plate and a suitable counter electrode comprising, for example, graphite or aluminum.
Conventionally known electrolytic etching methods, however, have been quite limited with respect to the types of electrolyzing solution compositions that can be used therewith, since a precise correlation exists between the electrolyzing conditions and the composition of the electrolyzing solution necessary to provide a uniformly roughened surface suitable as the support for planographic printing plate. Such correlation factors have often resulted in failure with respect to attempts to find a practically balanced combination of solution composition and electrolyzing condition.
An improved electrolytic surface etching method directed to partly solving this disadvantage occurring in the conventionally known technique is disclosed in U.S. Pat. No. 4,087,341, which describes an electrolytic etching process for an aluminum plate in an electrolytic comprising an aqueous hydrochloric acid or nitric acid solution, characterized by the application of voltage having a peak value for the anode phase higher than that for cathode phase, and an a.c. wave form such that the total current passed during the anodic phase is larger than the total current passed during the cathodic phase. An aluminum plate treated in accordance with this method has a roughened surface exhibiting a double pit structure, i.e., pits in pit structure. When a photosensitive coating is spread over such a surface-treated aluminum plate, a so-called presensitized planographic plate (referred to as a PS plate) can be obtained, which can be converted into printing plate of excellent performance.
However, in order to obtain planographic printing plates according to the above-described method, one cannot employ a readily available, commercial a.c. electrical source, but has to specially fabricate an a.c. wave form so as to meet the requirements cited above.